The separation of particulate impurities from industrial fluid streams is often accomplished using fabric filters. These textile based filter media remove particulate from the fluids. When the resistance to flow or pressure drop through the textile caused by accumulation of particulate on the filter becomes significant, the filter must be cleaned, and the particulate cake removed.
It is common in the industrial filtration market to characterize the type of filter bag by the method of cleaning. The most common types of cleaning techniques are reverse air, shaker and pulse jet. Reverse air and shaker techniques are considered low energy cleaning techniques.
The reverse air technique is a gentle backwash of air on a filter bag which collects dust on the interior. The back wash collapses the bag and fractures dust cake which exits the bottom of the bag to a hopper.
Shaker mechanisms clean filter cake that collects on the inside of a bag as well. The top of the bag is attached to an oscillating arm which creates a sinusoidal wave in the bag to dislodge the dust cake.
Pulse jet cleaning techniques employs a short pulse of compressed air that enters the interior top portion of the filter tube. As the pulse cleaning air passes through the tube venturi it aspirates secondary air and the resulting air mass violently expands the bag and casts off the collected dust cake. The bag will typically snap right back to the cage support and go right back into service collecting particulate.
Of the three cleaning techniques the pulse Jet is the most stressful on the filter media. However, in recent years industrial process engineers have increasingly selected pulse jet baghouses for dust collection applications because of:
1. Smaller unit size (sometimes as much as 1/2 or 1/4 the size of shakers and reverse air) due to PA1 2. Minimal number of moving parts. PA1 3. Lower number of bags to replace when failed.
(A) higher volumetric airflow/cloth area ratio (higher operating velocity through media) PA2 (B) on-line cleaning allows the unit to be designed at the design flow rate hence there is no need for additional filter media area to allow for off-line cleaning.
The need for high temperature (400.degree. F.), thermally stable, chemically resistant filter media in baghouses narrows the choice of filter media to only a few viable candidates for pulse jet applications. Common high temperature textiles comprise polytetrafluoroethylene (PTFE), fiberglass, or polyimides. When the effect of high temperature is combined with the effect of oxidizing agents, acids or bases, there is a tendency for fiberglass and polyimide medias to fail prematurely. Thus, there is a preferance for using PTFE. Commercially available PTFE fabrics are supported needlefelts of PTFE fiber. These felts usually weight from 20-26 oz/yd.sup.2 and are reinforced with a multifilament woven scrim (4-6 oz/yd.sup.2) The felts are made up of staple fibers, (usually 6.7 denier/filament, or 7.4 dtex/filament) and 2-6 inches in length. This product works similar to many other felted medias in that a primary dust cake "seasons" the bag. This seasoning, sometimes called in depth filtration, allows the media to filter more efficiently but has a drawback in that the pressure drop suffers. Eventually the bag will blind or clog and the bags will have to be washed or replaced. In general, the media suffers from low filtration efficiency, blinding and dimensional instability (shrinkage) at high temperatures.
In recent years, membrane laminate products have been used in filter applications involving pulse jet cleaning. A two layer product of porous expanded PTFE membrane laminated to woven porous expanded PTFE fiber fabric has been used. Commercial success of this product has not been realized due to several reasons, but primarily due to the woven fiber fabric backing not wearing well on the pulse jet cage supports. The woven yarns slide on themselves and create excessive stress on the membrane, resulting in membrane cracks.
The laminates of this invention are designed to solve these problems as well as provide the industrial filtration industry with the best filter media to solve high temperature, corrosive applications.